Communication systems that geographically reuse communication resources are known. These systems allocate a predetermined set of communication resources in one geographic area and reuse the same set of communication resources in one or more other geographic areas. This reuse technique improves communication capacity by minimizing the number of communication resources necessary to provide communication service in a large geographic area comprised of several smaller geographic areas. As is also known, a communication resource may be a frequency carrier, a pair of frequency carriers, a time slot in a time division multiplex (TDM) time frame, or any radio frequency (RF) transmission medium.
Two of the most common communication systems which geographically reuse communication resources are cellular communication systems and trunked mobile communication systems. In both communication systems, allocation of a communication resource begins when a communication unit requests communication service. Based on resource availability and signal usability, a resource controller assigns the communication resource to the communication unit. A communication, such as a conversation or a facsimile transmission, occurs on the communication resource between the communication unit and another communication unit or between the communication unit and a subscriber to a public service telephone network. The communication continues until completion or an interruption in service occurs. Upon conclusion of the communication, the resource controller retrieves the communication resource; thus making the communication resource available for another communication.
An important parameter in identifying an acceptable communication resource is signal usability. In a wireless communication system, the communication resources are typically RF channels which occupy predetermined bandwidths. When information signals are transmitted on the RF channels, undesired channel effects, such as fading and interference, alter the information signals during transmission. Thus, the information signals received by a receiver in the communication unit, or a base station, are corrupted by the undesired channel effects. By ascertaining an indication of the corruption on available communication resources, the least corrupted communication resource may be selected for the communication. This indication of corruption is known as signal usability.
In geographic reuse communication systems, signal usability is typically limited by the quantity of co-channel interference present on the RF channel. Co-channel interference occurs when receivers receive unwanted information signals from neighboring communication units, or base stations, transmitting on the same channel as the desired RF channel. Thus, the signal usability decreases as the co-channel interference increases.
Received signal strength indication (RSSI) and bit error rate (BER) are two common methods of estimating signal usability. In an RSSI estimate, the receiver measures the level of a received signal on the desired RF channel. This measurement provides a summation of signal levels (i.e. C+I+N) including the desired information signal (C), the co-channel interference (I), and the noise (N) on the desired RF channel. Although this technique accurately estimates the level of the received signal, it cannot distinguish between the desired information signal and signals due to co-channel interference. Thus, an acceptable RSSI measurement may provide unacceptable signal usability due to a high level of co-channel interference. Alternatively, BER measurements provide accurate estimates of signal usability, but in geographic areas where error rates are low, multiple measurements and excessive averaging times may be required to obtain the accurate estimates. Measurement periods as long as ten to fifty seconds may be necessary to obtain accurate BER data.
As briefly mentioned above, fading is an additional undesired channel effect that may alter the transmitted information signal. Fading occurs due to multiple reflections of the desired information signal during transmission over the RF channel. These reflections are typically caused by unintentional reflecting of the transmitted information signal from obstacles in its path, such as buildings and mountains, and may produce multiple modified replications of the transmitted information signal., each introducing various amplitude and phase alterations of the original signal in each new signal path. All of the transmitted information signal replicas form a composite information signal at the input to a receiver. The signal usability of the composite signal is dependant on the type of fading.
Two types of fading generally encountered are flat fading and frequency selective fading. In a digital transmission, flat fading occurs when the maximum differential time delay between each new signal path is much less than a symbol period. As is known, methods for estimating the flat fading of an RF channel exist and are used to minimize the degradation in signal usability attributed to flat fading. Frequency selective fading occurs when the maximum differential time delay between each new signal path is comparable to or greater than the symbol period. Frequency selective fading may also degrade signal usability. Recent technological advances have allowed signal quality to be estimated in the presence of frequency selective fading. For a detailed discussion of a method for measuring signal quality that accounts for frequency selective fading refer to U.S. Pat. No. 5,170,413, entitled "Control Strategy For Reuse System Assignments And Handoff", assigned to Motorola Inc. While this technology provides many advantages, it does not address the technological concern of estimating signal usability based on a measured approximation of co-channel interference and noise.
Therefore, a need exists for a method and apparatus that determine signal usability based on a determination of RF channel interference.